(57) An impedance blocking filter circuit is provided for use in telecommunication systems
for interconnecting between incoming telephone lines and customer's terminal equipment
so as to unconditionally block impedances above 20 KHz due to the customer's terminal
equipment from an ADSL network unit and/or home networking interface unit. The filter
circuit includes first, second, and third inductors (L5,L3,L1) connected in series
between a first input terminal (66) and a first common point (A). A first resistor
(R1) has its one end connected also to the first common point and its other end connected
to a first output terminal (70). Fourth, fifth and sixth inductors (L6.L4,L2) are
connected in series between a second input terminal (68) and a second common point
(B). A second resistor (R2) has its one end also connected to the second common point
and its other end connected to a second output terminal (72). A capacitor (C1) has
its ends connected across the first and second common points. In other aspects, the
filter circuit also includes switching means (K1,K2) for eliminating shunt additive
capacitance, correction circuit means (TC1,TC2) reducing significantly return loss,
and switch suppression circuit means (74) for eliminating transients.
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